the u.s. are expected to be on hand to help monitor the damaged reactors. you can also bet they'll have one of these, a geiger counter. the bbc's david shukman explains what they can expect when they get there. >> reporter: day after day, new threats emerge at fukushima, multiple explosions, sudden rises in radiation, a scene so hazardous only 50 workers are left on site to try to bring things under control. getting hard information about the fate of the power station is proving really difficult. here's what we think is happening. reactor one, its outer building exploded on saturday. there's probably been a partial melting inside. and they're still pumping in seawater. reactor two, the most worrying, with an explosion here last night and crucially a potential breach inside it. the first possible damage to any of the reactors. it's thought that this device down below, which handles excess pressure, may have been breached. and this could be one source of leaking radiation. now, reactor three is